I propose removing the callable() check[1] from typing._type_check. This restriction is usually met in typeform instances by implementing a __call__ method that raises at runtime[2]. _type_check is called at runtime in a few disparate locations, such as in an argument to typing.Annotated or for certain stringified annotations in typing.get_type_hints.

Because the requirement to be callable is unexpected and shows up in situations not easily discoverable during development or common typing usage, it is the cause of several existing cpython bugs and will likely continue to be the cause of bugs in typeforms outside of cpython. Known cpython bugs caused by the callable() check are bpo-46643, bpo-44799, a substantial contributing factor to bpo-46642, and partly bpo-46511. I discovered bpo-46643 with only a cursory check of typing.py while writing this proposal. Moreover, it doesn't make any particular technical sense to me why it should be required to add an awkward __call__ method.

Removing the callable() check fails 10 tests:
7 tests: checking that an int literal is not a type
2 tests: testing that list literals are not valid types (e.g. [3] raises a TypeError because the literal [('name', str), ('id', int)] doesn't pass callable())
1 test: bpo-46642

The responsibility of determining these invalid typeforms (e.g. int literals) would need to be passed to a static type checker. If it's desired to do this at runtime it's my opinion that a different check would be more appropriate.

Have I missed any reasons for the callable() check? Can I remove the check and adjust or remove the tests?

[1]

In addition to the 10 tests failed in test_typing.py, one additional test fails in test_types.py with this change:

This falls in category (1), checking that an int literal is not a type, but with the apparent intent to prevent index subscripting.

I also support this change. I've had to write a lot of code to make SpecialForms able to accept my types where the code has to look like:

class Something:
    ...
    def __call__(self, *args, **kwargs):
       raise NotImplementedError

I also know this comes up in typing-extensions a fair bit. I think type checkers should be enforcing this at type-checking-time not by typing.py run-time.

I agree with removing this check. I suspect it's a holdover from very early typing when static types were supposed to be runtime types. Now the check is a bug magnet and doesn't serve a useful purpose.

I think we can just remove the tests that check for ints. I don't see a principled reason to special-case int literals.

I wonder if we should apply this change to 3.10 and 3.9. It's arguably a bugfix, but it's a pretty big change.

Under the same failing int test cases before there were 2 more cases next to them that fail:

with self.assertRaises(TypeError):
    ClassVar[int, str]
with self.assertRaises(TypeError):
    Final[int, str]

These fail because tuple literals are not callable(). There is code that clearly intends for this to be the case:

I can either remove support for this runtime check or change the implementation of Final et al to reject tuple literals. I will do the latter for now.

For

Further questions: the msg argument in _type_check now wouldn't be used for anything! It was only used in the case where a type wasn't callable(). I think it should be removed. I'm also a bit negative on disallowing tuples in the case of e.g. Final and such since it complicates implementing tuple types in Python down the line if desired.

I made a draft pull request where I went ahead and added a check to disallow tuple literals. This is basically already disallowed for types by restrictions on __getitem__ because Union[typeform]->type needs to be different from Union[type,type]->Union[type,type].

There were two reasons of accepting arbitrary callables in _type_check():

1. NewType() returned a function.
2. xml.etree.cElementTree.Element was a function in Python 2.

Now NewType is a class, and Python 2 no longer supported. I agree that we should get rid of the callable() check, but I disagree with the proposed change. The check should be more explicit and strict, not more lenient. List[42] does not make sense and should not be accepted.

List[42] is already accepted, and your proposed patch does not change it to make it not accepted. The issue is _type_check is only called in a few particular locations; this is part of the technical reason I'm not very concerned about relaxing the _type_check requirements.

From a type checking philosophy point of view I agree with Jelle and am negative on strict runtime requirements in typing.py.

No, List[42] is not currently accepted.

>>> List[42]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 318, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1127, in __getitem__
    params = tuple(_type_check(p, msg) for p in params)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1127, in <genexpr>
    params = tuple(_type_check(p, msg) for p in params)
                   ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 184, in _type_check
    raise TypeError(f"{msg} Got {arg!r:.100}.")
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Parameters to generic types must be types. Got 42.

I'm referring to within type annotations, where this code path isn't used: try a: List[42]

This code path can show up in type aliases though.

But it gives the same result.

What version of Python do you test with?

I compiled your PR to run it and was testing in 3.10 as well, but I was testing in a file with from future import annotations unintentionally. I retract the comment. It turns out list[42] is okay though, which I suppose is more relevant going forward. My confusion here is sort of the crux of my problem with these runtime checks: they are inconsistently applied in different locations which is why callable() was causing a lot of bugs.

New changeset 870b22b by Gregory Beauregard in branch 'main':
bpo-46644: Remove callable() requirement from typing._type_check (GH-31151)
870b22b

Thanks for your contribution!

Some tests are failing on main, probably due to a race. PR incoming.

New changeset 7517437 by Jelle Zijlstra in branch 'main':
bpo-46644: Fix test_typing test broken by #75334 due to a merge race (GH-31833)
7517437